Ballast water means seawater carried as a weight for securing a draught to stabilize a ship, mainly when the ship is loaded with no cargo. When the ship is loaded with cargo, the cargo itself acts as a weight to stabilize a hull of the ship. However, when it is unloaded, the hull of the ship floats to decrease stability. Accordingly, a substantial amount of seawater is carried in a ballast tank after discharge of the cargo.
When the seawater is introduced into the ships as the ballast water for the ships, aquatic organisms are also introduced together. The aquatic organisms introduced are transported to distant regions with navigation of the ships. Then, the organisms are poured out to places different from their original habitat due to discharge of the ballast water. There is a possibility that the alien organisms thus poured out take root and bleed in the places different from their original habitat to cause disruption of ecosystem in such sea areas, damage to economic activities such as the fisheries industry, coast pollution and the like (see Non-Patent Document 1).
From such situations, it has been proposed to kill the alien organisms by adding biocidal agents such as sodium hypochlorite, aldehydes and alkynes to the ballast water introduced into the ships, as described in Patent Documents 1 to 3.
However, when the ballast water to which the biocidal agents have been added is discharged in large amounts, there is a possibility that native organisms in the sea areas to which the water has been discharged are killed. Accordingly, it has been proposed to render the ballast water harmless by adding reducing agents such as sodium sulfite and hydrogen or neutralizing agents such as amines, amino acids and amino alcohols thereto, before or at the time when the ballast water to which the biocidal agents have been added is discharged.